Monitoring and control of air flow in a ventilation system may be accomplished by creating a differential pressure signal based on volumetric flow rate. There are several industry standard techniques that include a Venturi, orifice plate, flow nozzle and a laminar flow channel. Once a differential pressure is created, a differential pressure sensor can be used to determine the volumetric flow rate.
Standard differential pressure sensors function by translating a pressure effect into a different effect that can be measured by standard means. Some of these other effects include changes in displacement, capacitance and resistance of a material. For example, a membrane-type sensor such as a piezoresistive sensor, changes its resistance in response to changes in pressure on a piezoresistive element. Such a sensor may be incorporated in a wheatstone bridge circuit that is well-suited to measure changes in resistance. Thus, a change in pressure may be detected as a change in resistance.
However, piezoresistive sensors have several shortcomings that are pronounced when applied to meet cost and size constraints of certain embedded applications. The reduction in size of a piezoresistive pressure sensor means a smaller area of the piezoresistive element. Since pressure equals force times area, a smaller area requires a greater pressure to generate the same force. Therefore, a smaller area will be displaced less than a larger area element when exposed to the same pressure. That translates into a reduction in the strain the piezoresistive element experiences as a function of pressure, which in turn translates into a reduction in signal strength. For a small area piezoresistive sensor having a small area piezoresistive element, changes in pressure may cause too small a deflection of the piezoresistive element change to be measurable.
Any sensor is useful only if the quantity being measured has a reasonable signal-to-noise ratio and produces a signal that can be differentiated from other quantities, such as changes in temperature, sensor orientation, humidity and material aging, to name a few, that can also effect the sensor signal. This is why the signal output cannot be continuously amplified to achieve any level of performance.
While low cost piezoresistive sensors are suitable for measuring large pressure and pressure differences, their performance suffers at low pressures (e.g., less than 1-inch water column). The effect on the sensor element due to pressure starts to compare with the effects due to other factors such as temperature, internal heating, material aging, mechanical stresses due to mounting, and stresses due to the different coefficient of expansions for the different materials that make up the sensor. All of these effects are difficult to account for when calibrating a pressure sensor. However, for pressure sensors used to detect small changes or differences in pressure, accurate calibration is critically important. Thus, piezoresistive elements may not be suitable as pressure sensors for certain embedded applications which require detection of small changes or differences in pressure.
It is thus desirable to provide a pressure sensor that overcomes the above-described shortcomings of the prior art.